The application of fiber-reinforced composite materials comprising reinforcing fibers and matrix resins is spreading in various fields, including sports and general industry, particularly aeronautics and space, as they enable material designs that best exploit the advantages of reinforcing fibers and matrix resins.
As reinforcing fibers, glass fiber, aramid fiber, carbon fiber, boron fiber and the like are used. As a matrix resin, both a thermosetting resin and thermoplastic resin can be used, though a thermosetting resin is more frequently used because of its ability to easily impregnate reinforcing fibers. Examples of a thermosetting resin include an epoxy resin, unsaturated polyester resin, vinyl ester resin, phenol resin, bismaleimide resin, and cyanate resin.
Fiber-reinforced composite materials are manufactured by applying the prepreg method, hand lay-up method, filament winding method, pultrusion method, resin transfer molding (RTM) method and other methods.
In recent years, Japanese as well as overseas automakers are striving to reduce the weight of vehicle bodies as a key parameter of fuel efficiency amid tightening environmental controls on motor vehicles around the world. The application of carbon-fiber composite materials, which only weigh about half and 70% as much as steel and aluminum, respectively, has been actively investigated. As well as weight reduction, various automobile structural parts are, in many cases, subject to high rigidity and high strength requirements, while often having complex three-dimensional shapes. This has made RTM a promising molding method because of its ability to accommodate complex shapes, combined with the use of high-rigidity high-strength continuous carbon fiber. With the RTM method, a fiber-reinforced composite material is obtained by placing a reinforcing-fiber base in a mold and, after the mold is closed, injecting a resin from a resin injection port or ports to impregnate the reinforcing fibers. After the resin is cured, the mold is opened to take out the molding. In this regard, productivity poses a major problem to the widespread use of carbon-fiber composite materials in motor vehicles and, because of this barrier, their adoption has not gone beyond some luxury cars.
With the hand lay-up method, filament winding method, pultrusion method and RTM method, a two-pack epoxy resin composition is often used from the viewpoint of moldability. A two-pack epoxy resin composition refers to an epoxy resin composition comprising a base resin pack containing an epoxy resin as the primary component and a curing agent pack containing a curing agent as the secondary component obtained by mixing the base resin pack and curing agent pack immediately before use. In contrast, an epoxy resin composition in which all the components, including the base resin and curing agent, are premixed is a one-pack epoxy resin composition. In the case of a one-pack epoxy resin composition, the curing reaction progresses even during storage, and this necessitates cold storage. In addition, a low-reactivity solid agent is often selected as the curing agent pack, and this makes it necessary to inject a one-pack epoxy resin composition into reinforcing fibers at high pressure using a press roll or the like to impregnate them. With a two-pack epoxy resin composition, on the other hand, a liquid agent can be chosen for both the base resin pack and curing agent pack, and this makes it possible to obtain a low-viscosity liquid epoxy resin composition with excellent reinforcing fiber impregnability as a mixture of a liquid base resin and liquid curing agent. Since the base resin pack and curing agent pack are stored separately, there are no specific limitations on storage conditions, and long-term storage is possible.
To realize high-level productivity as described above using the RTM method, for instance, it is not enough to just have a short resin curing time. Rather, there is also a specific need to simultaneously satisfy the four conditions listed below. First, the two packs have low and similar viscosity values and exhibit excellent mixability during the mixing to prepare a resin composition. Second, the resin composition exhibits low viscosity during impregnation of the reinforcing-fiber base and a rise in viscosity can be suppressed throughout this step to ensure excellent impregnability. Third, sufficiently fast curing is possible in the low temperature region around 100° C. This allows simplification of molding equipment and eliminates the need for heat resistance in secondary materials, thus giving rise to cost reductions, as well as excellent surface smoothness of moldings, achieved through a reduction in heat shrinkage attributable to the difference between the curing temperature and room temperature. Fourth, by the time of the demolding step, the resin has reached an adequate level of rigidity as a result of curing. This makes strain-free smooth demolding possible, while eliminating strain or deformation in the coating step, thus ensuring high dimensional accuracy in moldings.
To address these problems, an epoxy resin composition based on a combined use of an acid anhydride and phenol novolac as a curing agent has been disclosed. Since it suppresses generation of formalin and provides moldings with excellent rigidity, it is preferable as a sheet molding compound for building materials (Japanese Unexamined Patent Publication (Kokai) No. 2002-12649). However that material does not have adequate fast curing performance in the low temperature region.
Further, an epoxy resin composition based on a combined use of an acid anhydride curing agent and an organic phosphorus compound catalyst has been disclosed as an epoxy resin composition with an excellent balance between the length of the low viscosity period and that of curing time under constant temperature conditions around 100° C. (International Publication WO 2007/125759). However, that material has a problem in that it is inadequate in fast curing performance and lacks adequate resin rigidity during demolding, sometimes resulting in reduced dimensional accuracy.
In addition, an epoxy resin composition based on the use of a small amount of a carboxylic acid anhydride as an auxiliary catalyst in combination with a phenolic curing agent has been disclosed as an epoxy resin composition for electrical materials that has simultaneously achieved high heat resistance, high toughness and high adherence to copper foils (Published Japanese Translation of PCT International Publication JP 2009-521566). However, that material also fails to have adequate fast curing performance, while being unsuitable for molding applications due to its need for a solvent.
As can be seen from the above, a two-pack epoxy resin composition that accommodates high-cycle molding (a large number of molding cycles in a given amount of time, made possible by reducing the length of time required to execute a single molding cycle), particularly RTM, and simultaneously meets all the requirements set to realize high productivity does not yet exist.
It could therefore be helpful to provide a two-pack epoxy resin composition that offers excellent workability during resin preparation, maintains low viscosity during impregnation of reinforcing fibers, thus exhibiting excellent impregnability, is quickly cured during molding and provides fiber-reinforced composite materials with high dimensional accuracy, a curing agent pack, and a fiber-reinforced composite material produced therefrom.